Lift and Drag Monitor Point Values Converging to Zero

Josh · November 23, 2010, 20:01

It's been months since I posted this, but I still suffer from the same problems. My friction and pressure graphs match the published data very well, yet the function calculator in CFX incorrectly predicts the lift and drag. Notably, it often predicts a negative drag value, whether using the "force" or "forceNorm" functions.

I reported this possible bug to ANSYS in August and never received a response. It's irritating because I have to manually perform my lift and drag calculations, plus my lift and drag history plots aren't accurate in magnitude.

Does anyone else suffer from this problem? Has it been fixed in v13?

icemaniac178 · May 5, 2011, 22:36

dear josh,
i have gone through your post and i believe i have the same problem where my lift and drag value are too low and not reasonable. i also suspect there is some problem with the `force` function in the CFX calculator. is there any adjustment that u make in the CFX calculation? how do you apply the proper calculation in your works?

Josh · May 6, 2011, 16:07

I was changing my inlet velocity angle to simulate angles of attack. That caused problems. I used the following thread to solve my problem:
http://www.cfd-online.com/Forums/mai...oefficent.html

icemaniac178 · May 9, 2011, 01:18

thank you mr josh,

do you mean that u used this formula to simulate the AOA variation?

L=Fy cos(alpha) - Fx sin(alpha) D=Fx cos(alpha) + Fy sin(alpha)

actually i have already adopted this formula to simulate the AOA variation.
however, my lift and drag ceofficient is still very low even at high AOA (20 degrees).
i`m thinking about changing the wing mesh rotation in order to simulate the AOA variation. is it the best technique? or are there any other better ways to do it?

Josh · May 9, 2011, 11:38

That's the formula I use.

As for your problem, there are many, many possible causes. Grid resolution, turbulence model, domain size, timestepping scheme...

You'll have to try some sensitivity analyses to figure out the problem.

Good luck.

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November 23, 2010, 20:01		#21
Josh Senior Member Joshua Counsil Join Date: Jul 2009 Location: Halifax, Nova Scotia, Canada Posts: 366 Rep Power: 18	It's been months since I posted this, but I still suffer from the same problems. My friction and pressure graphs match the published data very well, yet the function calculator in CFX incorrectly predicts the lift and drag. Notably, it often predicts a negative drag value, whether using the "force" or "forceNorm" functions. I reported this possible bug to ANSYS in August and never received a response. It's irritating because I have to manually perform my lift and drag calculations, plus my lift and drag history plots aren't accurate in magnitude. Does anyone else suffer from this problem? Has it been fixed in v13?

May 5, 2011, 22:36		#22
icemaniac178 Member iswadi Join Date: Feb 2010 Posts: 44 Rep Power: 16	dear josh, i have gone through your post and i believe i have the same problem where my lift and drag value are too low and not reasonable. i also suspect there is some problem with the `force` function in the CFX calculator. is there any adjustment that u make in the CFX calculation? how do you apply the proper calculation in your works?

May 6, 2011, 16:07		#23
Josh Senior Member Joshua Counsil Join Date: Jul 2009 Location: Halifax, Nova Scotia, Canada Posts: 366 Rep Power: 18	I was changing my inlet velocity angle to simulate angles of attack. That caused problems. I used the following thread to solve my problem: http://www.cfd-online.com/Forums/mai...oefficent.html

May 9, 2011, 01:18		#24
icemaniac178 Member iswadi Join Date: Feb 2010 Posts: 44 Rep Power: 16	thank you mr josh, do you mean that u used this formula to simulate the AOA variation? L=Fy cos(alpha) - Fx sin(alpha) D=Fx cos(alpha) + Fy sin(alpha) actually i have already adopted this formula to simulate the AOA variation. however, my lift and drag ceofficient is still very low even at high AOA (20 degrees). i`m thinking about changing the wing mesh rotation in order to simulate the AOA variation. is it the best technique? or are there any other better ways to do it?

May 9, 2011, 11:38		#25
Josh Senior Member Joshua Counsil Join Date: Jul 2009 Location: Halifax, Nova Scotia, Canada Posts: 366 Rep Power: 18	That's the formula I use. As for your problem, there are many, many possible causes. Grid resolution, turbulence model, domain size, timestepping scheme... You'll have to try some sensitivity analyses to figure out the problem. Good luck.